In Skyrim, when I first arrived at Forelhost, there was this Imperial alone by his firecamp. I tried to enter Forelhost, but the door was locked and absolutely needed a key. I talked to the Imperial camping just by the door but he didn't mention Forelhost.
I tried to pick-pocket the Imperial, hoping that he had the key and saw that note that tells that he is an impostor! Then he got mad for the pick-pocket and tried to kill me, so I killed the false-Imperial.
I entered Forelhost, crawled the dungeon, learned its gruesome story and picked up the great unique item at the end. Suprise! A new quest appears: "Siege on the Dragon Cult" with that objective: Return to Captain Valmir.
I got outside via the balcony and saw an Imperial standing beside the impostor's body. I said to myself: "Hey! Here's the real Captain Valmir! Time to tick that quest!". To my great surprise, as soon as I got near he attacked and I had to kill him in a fiery burst of flames.
But now the quest "Siege on the Dragon Cult" stays there, waiting for me to return to the Captain Valmir that is now a roasted to a crisp by the door to Forelhost, not far from the body of the impostor.
How can I complete that quest? (I am using the PC version of Skyrim and can use the console if needed.)